Dynamic biasing circuit for continuous time comparators

ABSTRACT

A method for dynamically adapting the biasing current for a fast switching voltage comparator is achieved. The difference of the two input signals of said comparator controls the comparator&#39;s biasing current, where the biasing current is high only when the difference is low and the comparator&#39;s switching is likely to happen and where the biasing current is kept low at other times. In a current mirroring circuit, the voltage difference at the comparator inputs controls the mirroring ratio. The biasing current reaches its maximum when the input voltage difference approaches zero. Once the input voltage difference crosses zero and continues to change in the same direction as before, that is after the polarity of the voltage difference changed, the control mechanism alternates the connection of the input signals to the current controlling elements, in order to now reduce the current with a further increase of the voltage difference. The same circuit can have opposite characteristics, providing a minimum current when the input voltage difference is low, by reversing the connection of the inputs to the current controlling elements. An alternative circuit is described, that does not need said alternating mechanism.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The invention relates to a fast switching continuous time CMOS comparator with dynamic biasing circuit, and more particularly, to a continuous time comparator having a dynamic biasing circuit which self-adjusts the biasing current, depending on the voltage difference of the two input signals of the comparator, in order to draw high biasing current only when required and still maintaining overall low power consumption.

[0003] (2) Description of the Prior Art

[0004] Continuous time CMOS comparators use high bias currents to achieve high speed switching characteristics. Since it is unknown when the switching point is being reached, bias current is always left at its high level, causing high overall power consumption.

[0005] Several patents describe the use of dynamic or adaptive or self-adjusting bias currents, however, none of them uses such an adaptive method to get high bias currents at the time when switching is likely to happen and to reduce said bias current at other times.

[0006] U.S. Pat. No. 6,204,654 (Miranda, et al.) dicloses a dynamically boosted current source circuit to improve the response speed of a circuit responsive to a transitioning input signal. A threshold detector detects the occurrence of an input signal transition prior to its detection by the responsive circuit, and triggers the current source to provide the boosted current; this improves the responsive circuit's speed. The current source provides the boosted current for a predetermined time interval, or until the input signal crosses a second threshold, enabling response speed to be increased without a significant increase in supply current.

[0007] U.S. Pat. No. 5,471,171 (to Itakura, et al.) describes an amplifier device capable of realizing high slew rate with low power consumption. It includes a determination element for determining a change condition of the level change of the input signal to generate a control signal corresponding to said change condition and a driving capability increase element as a circuit for increasing a bias current, depending on said change condition, in order to increase the driving output supplied.

[0008] U.S. Pat. No. 6,239,658 (to Tham) shows a control circuit, which is used to increase the DC, current of a low noise amplifier for a mobile wireless communication system in the presence of jamming signal to preserve high dynamic range. The difference signal between the input signal with jamming signal and the average input is detected by a peak detector.

[0009] The following articles discuss subjects close to the invention:

[0010] “Adaptive Biasing CMOS Amplifiers”, (Degrauwe, Rijmenants, Vittoz and De Man, IEEE Journal of Solid-State Circuits, Vol SC-17, No. 3 June 1982 pp 887) discusses two transconductance amplifiers in which the concept of an input dependent bias current is introduced.

[0011] “Dynamic CMOS Amplifiers”, (Hosticka, IEEE Journal of Solid-State Circuits, Vol SC-15, No. 5 October 1980, pp 522) presents the concept of dynamic amplifiers with different biasing principles.

SUMMARY OF THE INVENTION

[0012] A principal object of the invention is to reduce the current consumption of a fast switching comparator. A basic requirement is however to supply a high biasing current at least at the time when switching actually takes place, in order to maintain the required fast switching speed.

[0013] In accordance with the objectives of this invention, a circuit to feed a high biasing current into said comparator only at the time when switching is likely to happen, and significantly reducing the biasing current at all other times, while maintaining the fast switching speed, is achieved. The circuit comprises a fast CMOS comparator, a reference current source, the means to mirror the reference current into a second current branch and the means to feed this current into said comparator circuit as the comparator's biasing current. The current mirrored into the second branch may be either equal to or a predefined multiple of the reference current. Furthermore it comprises the means to modify the current mirroring ratio with voltage controlled resistive elements, depending on said input signals' voltage difference. Furthermore it provides the connection from said input signals to the circuit with the resistive elements, which modifies the current mirroring ratio in a way, that the current is reduced, when said input signals' voltage difference increases. In addition it provides the mechanism, to alternate the connection of the two input signals to the proper branch of said circuit with the resistive elements, which modifies the mirroring ratio of said current mirror and it provides the means to select the status of said alternating mechanism.

[0014] More specific, a circuit comprises a fast CMOS comparator, a reference current source, a current mirror, typically built by pairs of transistors, to mirror the reference current into a second current branch and a connection to feed this current into said comparator circuit as the comparator's biasing current. The current mirrored into the second branch may be either equal to or a predefined multiple of the reference current. Furthermore it comprises a first voltage controlled resistive element to modify the current mirroring ratio, in series with one of the said current mirror branches and controlled by one of the comparator's inputs and a second voltage controlled resistive element to modify the current mirroring ratio, in series with the other of the said current mirror branches and controlled by the other of the comparator's inputs. Furthermore it provides the connection from said input signals to the circuit with the resistive elements which modifies the current mirroring ratio in a way, that the current is reduced, when said input signals' voltage difference increases. In addition it provides a matrix of switching elements to alternate the connection of the two input signals to the proper branch of said circuit with the resistive elements, which modifies the mirroring ratio of said current mirror and it provides the means to select the status of said a matrix of switching elements.

[0015] In accordance with the objectives of this invention, a method to feed a high biasing current into said comparator only at the time when switching is likely to happen, and significantly reducing the biasing current at all other times, is achieved. First a reference current is mirrored into a second branch; its current provides the biasing current for the CMOS comparator. Two resistive elements in series with the two current mirror branches modify the current mirroring ratio, thus changing the biasing current. The two resistive elements are controlled by the two input signals of the comparator: when the voltage difference of the two input signals is small, the biasing current will be equal to or a predefined multiple of the mirrored reference current. Furthermore, to achieve the objectives of this invention, a method to connect the one of said two resistive elements with one of said inputs of the comparator and to connect the other of said two resistive elements with the other of said inputs of the comparator, is implemented in a way, that the higher voltage difference at the inputs will reduce the current in this branch. After the input signal crossed the zero difference point and after the comparator switched, a control mechanism alternates the connection between the two input signals and the resistive elements. The effect is, that the biasing current reaches its maximum value when the input voltage difference is zero; from that point on, when the voltage difference at the two inputs becomes larger again, the biasing current is now reduced.

[0016] Under specific conditions the opposite characteristics of the current biasing circuit may be desired: the biasing current should be high when the input signal voltages are far apart and the current should be low when the input signal voltages are close together. Another object of the invention is therefore to feed a high biasing current into a comparator only at the time when the difference of the input signal voltage is high and to keep the biasing current low, when the input signal voltages are close together.

[0017] In accordance with this specific objective of the invention, the same circuit as described above is used, where the connection from the inputs of the comparators to the current controlling branch however is reversed. Now a circuit to feed a high biasing current into a comparator only at the time when the difference of the input signal voltage is high and to keep the biasing current low, when the input signal voltages are close together, is achieved. This circuit again, provides a CMOS voltage comparator, means to mirror the reference current into a second current branch and feeding this said current into said comparator circuit as its biasing current, means to modify the current mirroring ratio depending on said input signals' voltage difference, means to connect said input signals to the circuit, which modifies the current mirroring ratio in a way, that the current increases, when said input signals' voltage difference increases. Furthermore it provides the means to alternate which of the two input signals is connected to which branch of said circuit, which modifies the current mirroring ratio of said current mirror and means to control the status of said alternating mechanism for said two input signals and the two said branches of the current mirroring circuit.

[0018] More specific, the circuit comprises a fast CMOS comparator, a reference current source, a current mirror, typically built by pairs of transistors, to mirror the reference current into a second current branch and a connection to feed this current into said comparator circuit as the comparator's biasing current. The current mirrored into the second branch may be either equal to or a predefined multiple of the reference current. Furthermore it comprises a first voltage controlled resistive element to modify the current mirroring ratio, in series with one of the said current mirror branches and controlled by one of the comparators inputs and a second voltage controlled resistive element to modify the current mirroring ratio, in series with the other of the said current mirror branches and controlled by the other of the comparator's inputs. Furthermore it provides the connection from said input signals to the circuit with the resistive elements which modifies the current mirroring ratio in a way, that the current increases, when said input signals' voltage difference increases. In addition it provides a matrix of switching elements to alternate the connection of the two input signals to the proper branch of said circuit with the resistive elements, which modifies the mirroring ratio of said current mirror and it provides the means to select the status of said a matrix of switching elements.

[0019] To achieve the objectives of this invention with said opposite characteristics for the current versus input voltage difference, a method to connect the one of said two resistive elements with one of said inputs of the comparator and to connect the other of said two resistive elements with the other of said inputs of the comparator, is implemented in a way, that the higher voltage difference at the inputs will increase the current in this branch. The resulting biasing current versus the input voltage difference now has a minimum value, when the input signals cross the zero voltage difference point.

[0020] Another circuit, with a different structure, but very similar to it in its major characteristics, is disclosed in this invention. A multiple of current controlling elements is introduced. Then a switching matrix like in the first of the circuits mentioned is not required. A major advantage of a switchless solution is to avoid switching noise. Achievable power saving might be less. The circuit, according to this invention, comprises a CMOS comparator, a reference current source, the means to mirror the reference current into a second current branch and the means to feed this current into said comparator circuit as the comparator's biasing current. Furthermore it comprises the means to modify the current mirroring ratio: one group of these means is in series with one branch of said current mirror and is controlled by signals from both inputs of said comparator, another group of these means is in series with the other branch of said current mirror and is again controlled by signals from both inputs of said comparator. Furthermore it provides the connection from said input signals to the circuit with the resistive elements, which modifies the current mirroring ratio. The resulting biasing current versus the input voltage difference again has a maximum value, when the input signals cross the zero voltage difference point.

[0021] More specific, the circuit comprises a fast CMOS comparator, a reference current source, a current mirror, typically built by pairs of transistors, to mirror the reference current into a second current branch and a connection to feed this current into said comparator circuit as the comparator's biasing current. The current mirrored into the second branch may be either equal to or a predefined multiple of the reference current. Furthermore it comprises a first resistive element to modify the current mirroring ratio, in series with one branch of the said current mirror and connected with one of the comparator's inputs and a second resistive element, parallel to the first resistive element, to modify the current mirroring ratio, in series with the same branch of said current mirror and connected with the other of the comparator's inputs and a third resistive element to modify the current mirroring ratio, in series with the second branch of said current mirror and connected with one of the comparators inputs and a forth resistive element, in series with the third resistive element, to modify the current mirroring ratio, in series with the second branch of said current mirror and connected with the other of the comparator inputs. Furthermore it provides the connection from said input signals to the circuit with the resistive elements which modifies the current mirroring ratio in a way, that the current is reduced, when said input signals' voltage difference increases.

[0022] To achieve the objectives of this invention, a method to feed a high biasing current into said comparator only at the time when switching is likely to happen, and significantly reducing the biasing current at all other times, is achieved. First a reference current is mirrored into a second branch; its current provides the biasing current for the CMOS comparator. Furthermore a method to modify the current mirroring ratio is through circuit components, operating as four groups of resistive elements: two groups; in a parallel arrangement and in series with one branch of said current mirror are controlled by either input of the said comparator and two groups in a serial arrangement and in series with the other branch of said current mirror are again controlled by either input of the said comparator. Furthermore, to achieve the objectives of this invention, methods to connect the said comparators inputs with the groups of resistive elements are implemented. Changes in the input signals cause changes in the current mirror ratio; the current mirror is in balance when the voltage difference of the two input signals is small. The effect is, that the biasing current reaches its maximum value when the input voltage difference is zero; from that point on, when the voltage difference at the two inputs becomes larger again, the biasing current is now reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the accompanying drawings, forming a material part of this description, there is shown:

[0024]FIG. 1 is a basic circuit diagram of the comparator, together with the elements to generate and to control the biasing current.

[0025]FIGS. 2a and 2 b show the resulting biasing current I₂ versus the input voltage difference of signals V₁ and V₂, when the switching sequence is selected to produce a maximum current for narrow signals V₁ and V₂.

[0026]FIGS. 3a and 3 b show the resulting biasing current I₂ versus the input voltage difference of signals V₁ and V₂, when the switching sequence is selected to produce a minimum current for narrow signals V₁ and V₂.

[0027]FIG. 4 shows an alternative circuit diagram, very similar in function as the circuit of FIG. 1, where the biasing current is a maximum when the input voltages approach the zero difference.

[0028]FIG. 5 shows the method to control the biasing current as a flow chart.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Continuous time CMOS comparators use high bias currents to achieve high speed switching characteristics. They draw high current even if the operating conditions make it very unlikely that switching may happen. The most important parameter is the switching speed; which directly depends on the biasing current at the time of switching. However, at times where switching is very unlikely to happen, the biasing current could be reduced; the overall power consumption would then be significantly lower. The preferred embodiments disclose a novel method and circuit to feed a high biasing current to said CMOS comparator only when switching is likely to happen and reduces said biasing current significantly at other times.

[0030] As switching of a comparator will only happen when the voltage difference of the two input signals of the comparator is low and switching is unlikely to happen when the voltage difference of the two input signals of the comparator is high.

[0031] A controllable current source will feed the biasing current into the comparator, where the current of said current source would be controlled by said circuit determining the voltage difference of said comparator inputs. Such current source may be represented by a current mirror, where the mirroring ratio can be varied depending on the input voltage difference.

[0032] According to the objectives of the invention, the biasing current should have its maximum just before zero crossing of the two input signals. Once the input voltage difference crosses zero and continues to change in the same direction as before, so that the polarity of the voltage difference at the comparator inputs now has changed, the control mechanism of the current mirror must alternate its operation in order to now reduce the current with a further increase of the voltage difference. For this purpose the connection within the control mechanism from the input signals to the current mirror branches will be alternated

[0033] A current mirror forms the heart of the circuit In the original concept of current mirrors, with matched pairs of components, they mirror a reference current into an output current of equal value. However with appropriate means they can be designed to mirror the current as a defined multiple of the reference current. A current mirror may, for example, be built of a matched transistor pair and a reference current source.

[0034] Voltage controlled resistive elements in series with each branch of the current mirror change the characteristics of the current mirror. The two branches of said current mirror and the two voltage controlled resistive elements, in series with each branch of the current mirror, can be considered to build a bridge circuit. These resistive elements are directly controlled by the two input signals of the comparator, in a way, that, when an input signal changes relative to the other, the balance of the current mirror will change, specifically, when the voltage difference of both signals get smaller, said biasing current will increase. However when both input signals change in common mode, the current mirror's balance remains unchanged.

[0035]FIG. 1 shows a reference current source I_(ref) and a current mirror, built by the matched transistor pair N₁ and N₂, which receives the reference current as current I₁ in one branch and mirrors it into the other branch as I₂.

[0036] The two transistors N3 and N4 in FIG. 1 operate as voltage controlled resistors and change the characteristics of the current mirror. Either gate of these transistors is connected to the input signals V₁ and V₂. Such connection may be a direct connection or through some level-shifting means.

[0037] The following functional description assumes circuit according to FIG. 1, where NMOS-transitors are used for all FET. If however PMOS-transistors should be used, all voltage levels will be reversed.

[0038] If the larger one of the two input signals is connected to N₃ and the smaller one is connected to N₄, then current I₂, which is the biasing current into the comparator, is smaller than I₁. If the input signals approach each other, I₂ increases in an almost linear way. For the input signals V₁ equal V₂, the currents I₁ and I₂ become equal, too.

[0039] If the input signals continue to change in the same direction as before, then I₂ would further increase. Therefore shortly after the two input signals crossed zero difference and after the comparator output switched, the connection of the gates of transistors N₃ and N₄ will be alternated. After alternating, with further increase of the input voltage difference the current I₂ will decrease. Again N₃ is connected to the larger input signal.

[0040] If the larger input signal is always connected to N₃ an effective “current boost” is realized during the switching event of the comparator. This characteristic of the biasing current versus input voltage difference is shown in FIG. 2a and FIG. 2b. When the input voltage V₁ and V₂ change over time t like in FIG. 2a, then the biasing current 12 changes over time t as shown in FIG. 2b. The figure also shows the currents I₁ and I₂ being “in balance”, at the point when the input voltages V₁ and V₂ cross each other.

[0041] Transistors N₃ and N₄ serve two major purposes in the circuit as described in this invention: they are the elements to control the current mirror ratio and they are, at the same time, the sensing elements for the input signal difference, thus being able to achieve the desired circuit function of significant biasing current reduction with a minimum of components. However N₃ and N₄ may also be built by a more complex arrangement of components.

[0042] The method to achieve the objectives of this invention is illustrated in FIG. 5. First, in step 51, the circuit mirrors the reference current into a second current branch. Then in step 52 said second current branch feeds the mirrored current into the comparator circuit as the comparators biasing current. It connects the input signals to the means, that modify the current mirroring ratio in step 53. If the input voltage difference becomes smaller (step 54) the current mirroring ratio is modified to increase the biasing current in step 55. If the input voltage difference becomes larger instead (step 54) the current mirroring ratio is modified to reduce the biasing current in step 56. The comparator circuit compares the voltage of said two input signals (step 57) and as soon as the comparator input signals cross the zero difference point (step 58), it switches the output according to the input's polarity and then, in step 59, the circuit alternates the connection of each of the two input signals to the means, that modify the current mirroring ratio.

[0043] In addition to the above invention, the preferred embodiments also disclose a novel method and circuit to feed a high biasing current to said CMOS comparator only when the voltage difference of the two inputs is large and reduces said biasing current significantly at other times. The behaviour of this embodiment is opposite to the behaviour, as described before: when the smaller input signal is always connected to N₃, an effective “current boost” is realized when the voltage difference of the input signals to the comparator is large. This characteristic is shown in FIG. 3a and FIG. 3b. To achieve this opposite behavior, said switches to alternate the connection of V₁ and V₂ with N₃ and N₄ are just controlled in the opposite way as described above.

[0044] Another embodiment to achieve the first described objective is shown in FIG. 4. The controllable current source (same as in FIG. 1) will feed the biasing current into the comparator, where the current of said current source would be controlled by a circuit on the basis of the voltage difference of said comparator inputs.

[0045] Biasing current should again have its maximum when the voltage difference of the two input signals is low, regardless of the polarity of the difference. This characteristic of the biasing current versus input voltage difference is similar to FIG. 2a and FIG. 2b.

[0046] As in the circuit described before, the current mirror forms the heart of the circuit and mirrors a reference current into an output current, either of equal value, or with a defined multiple of the reference current.

[0047] Again, voltage controlled resistive elements in series with each branch of the current mirror change the characteristics of the current mirror. These resistive elements are directly controlled by the two input signals of the comparator, in a way, that, when an input signal changes relative to the other, the balance of the current mirror will change, specifically, when the voltage difference of both signals gets smaller, said biasing current will increase. However when both input signals change in common mode, the current mirror's balance remains unchanged.

[0048]FIG. 4 shows a reference current source Iref and a current mirror, built by the matched transistor pair N_(1B) and N_(2B), which receives the reference current as current I_(1B) in one branch and mirrors it into the other branch as I_(2B).

[0049] In the circuit of FIG. 4, the FET transistor N₃ of FIG. 1 is replaced by two FET transistors N_(3B) and N_(4B) connected in parallel and the FET transistor N₄ of FIG. 1 is replaced by two FET transistors N_(5B) and N_(6B) connected in series. All FET transistors N_(3B) through N_(6B) operate as voltage controlled resistors. They change the mirroring ratio of the current mirror. The gates of transistors N_(3B) and N_(5B) are connected to the input signal V₁B and the gates of transistors N_(4B) and N_(6B) are connected to the input signal V_(2B).

[0050] Transistor pairs N_(3B) and N_(4B), connected in parallel, and transistor pairs N_(5B) and N_(6B), connected in series, serve two major purposes in the circuit as described in this invention according to FIG. 4: they are the elements to control the current mirror ratio and they are, at the same time, the sensing elements for the input signal difference, thus being able to achieve the desired circuit function of significant biasing current reduction with a minimum of components. However transistor pairs N_(3B)/N_(4B) and N_(5B)/N_(6B) may also be built by more complex arrangements of components.

[0051] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A circuit to supply the biasing current for a fast switching comparator with overall reduced power consumption and where the biasing current has its maximum when the voltage difference of the input signal is low, comprising: means to compare the voltage of two input signals, where the output switches according to the polarity of said two input signals' difference; a reference current source; means to mirror the reference current into a second current branch; means to feed the current of said second branch of the current mirror into said comparator circuit as the comparator's biasing current; means to modify the current mirroring ratio, depending on said input signals' voltage difference; means to connect said input signals to the circuit, which modifies the current mirroring ratio in a way, that the current is reduced, when said input signals' voltage difference increases; means for an alternating mechanism, which selects, which of the two input signals is connected to which branch of said circuit, which modifies the current mirroring ratio of said current mirror; and means to select the status of said means of alternating the connection between said two input signals and the two said branches of the current mirroring circuit, based on the output status of said comparating means.
 2. The circuit of claim 1 wherein said means to compare the two input signals is an integrated voltage comparator circuit.
 3. The circuit of claim 1 wherein said means to compare the two input signals is an integrated CMOS comparator circuit.
 4. The circuit of claim 1 wherein said means to mirror the reference current is built by an arrangement of FET-Transistors.
 5. The circuit of claim 1 wherein a first voltage controlled resistive element to modify the current mirroring ratio is in series with one branch of said current mirror and wherein a second voltage controlled resistive element to modify the current mirroring ratio is in series with the other branch of said current mirror.
 6. The circuit of claim 5 wherein said means to modify the current mirroring ratio is built by an arrangement of FET-Transistors, which operates as a voltage controlled resistive element.
 7. The circuit of claim 1, wherein the larger one of the two said input signals connects to one of the circuits, building a voltage controlled resistive element and in series with one of the current mirror branches, and wherein the smaller one of the two said input signals connects to the other of the circuits, building a voltage controlled resistive element and in series with the other branch of the current mirror.
 8. The circuit of claim 1 wherein said means to alternate which of the two input signals is connected to which branch of said circuit, modifying the current mirroring ratio, is directly dependent of said voltage comparator's output status.
 9. The circuit of claim 1 wherein said second branch of the current mirroring circuit directly feeds the biasing current to said integrated comparator circuit.
 10. A circuit to supply a high biasing current for a fast switching only when the input voltage difference is low, comprising: a CMOS comparator to compare the voltage of two input signals and switch the output according to the polarity of said two input signals' difference; a reference current source; a current mirror, typically built by pairs of transistors, to mirror the reference current into a second current branch; a connection to feed the current of said second branch of the current mirror into said comparator circuit as the comparator's biasing current; a first voltage controlled resistive element to modify the current mirroring ratio, in series with one of the said current mirror branches and controlled by one of the comparator's inputs and a second voltage controlled resistive element to modify the current mirroring ratio, in series with the other of the said current mirror branches and controlled by the other of the comparator's inputs; a connection of said input signals to the circuit, which modifies the current mirroring ratio; a matrix of switching elements, to alternate the connection between said two input signals and the proper branch of said voltage controlled resistors, which modifies the current mirroring ratio of said current mirror; and a circuit to select the status of said switch matrix, to alternate the connection between said two input signals and the proper branch of said voltage controlled resistors, based on the output status of said comparating means.
 11. The circuit of claim 10, wherein the voltage controlled resistive elements to modify the current mirroring ratio are formed by an arrangement of FET transistors.
 12. The circuit of claim 10, wherein the matrix of switching elements is formed by an arrangement of FET transistors.
 13. A method of feeding high biasing current to a fast switching CMOS comparator circuit only when the input voltage difference is low, comprising: providing a CMOS voltage comparator, means to mirror the reference current into a second current branch and feeding this said current into said comparator circuit as its biasing current, means to modify the current mirroring ratio depending on said input signals' voltage difference, means to connect said input signals to the circuit, which modifies the current mirroring ratio, means to alternate which of the two input signals is connected to which branch of said circuit, which modifies the current mirroring ratio of said current mirror and means to control the status of said alternating mechanism for said two input signals and the two said branches of the current mirroring circuit; comparing the voltage of two input signals with a CMOS circuit and switching the output according to the polarity of said two input signals difference; mirroring said reference current into a second current branch which has either a one to one relationship to said reference current or which otherwise has a fixed ratio to the reference current; feeding said mirrored current into said comparator circuit as the comparators biasing current; modifying the current mirroring ratio, in order to increase said biasing current when said input signals' voltage difference becomes smaller and to reduce the biasing current when said input signals' voltage difference becomes larger; producing an unmodified mirroring ratio when the voltage difference of both input signals is zero, wherein at this point the current mirror is defined to be in balance and where said mirrored current is equal to the desired maximum of said biasing current; connecting said input signals to the means that modify the current mirroring ratio depending on said input signals' voltage difference; alternating said connection of each of the two input signals to the proper branch of the means, that modifies the current mirroring ratio, depending on the polarity of said input signals' voltage difference; and selecting the proper status of said connection between said two input signals and the two said branches of the current mirroring circuit.
 14. The method of claim 13 wherein increasing said current mirroring ratio is achieved by increasing the resistance of said first voltage controlled resistive element, which is in series with one branch of said current mirror and by reducing the resistance of said second voltage controlled resistive element, which is in series with the other branch of said current mirror.
 15. The method of claim 13 wherein reducing said current mirroring ratio is achieved by reducing the resistance of said first voltage controlled resistive element, which is in series with one branch of said current mirror and by increasing the resistance of said second voltage controlled resistive element, which is in series with the other branch of said current mirror.
 16. The method of claim 15 wherein both voltage controlled resistive elements, which are in series with said two branches of said current mirror, are in balance at the point in time, where the voltage difference of said comparator reaches zero and where said mirrored current is equal to the desired maximum of said biasing current.
 17. A circuit to supply the biasing current for a comparator with overall reduced power consumption and Where the biasing current has its maximum when the voltage difference of the input signal is high and has its minimum when the voltage difference of the input signal is low, comprising: means to compare the voltage of two input signals, where the output switches according to the polarity of said two input signals' difference; a reference current source; means to mirror the reference current into a second current branch; means to feed the current of said second branch of the current mirror into said comparator circuit as the comparator's biasing current; means to modify the current mirroring ratio, depending on said input signals' voltage difference; means to connect said input signals to the circuit, which modifies the current mirroring ratio in a way, that the current increases, when said input signals' voltage difference increases; means for an alternating mechanism, which selects, which of the two input signals is connected to which branch of said circuit, which modifies the current mirroring ratio of said current mirror; and means to select the status of said means of alternating the connection between said two input signals and the two said branches of the current mirroring circuit, based on the output status of said comparating means.
 18. The circuit of claim 17 wherein said means to compare the two input signals is an integrated voltage comparator circuit.
 19. The circuit of claim 17 wherein said means to mirror the reference current is built by an arrangement of FET-Transistors.
 20. The circuit of claim 17 wherein a first voltage controlled resistive element to modify the current mirroring ratio is in series with one branch of said current mirror and wherein a second voltage controlled resistive element to modify the current mirroring ratio is in series with the other branch of said current mirror.
 21. The circuit of claim 17 wherein said means to alternate which of the two input signals is connected to which branch of said circuit, modifying the current mirroring ratio, is directly dependent of said voltage comparator's output status.
 22. The circuit of claim 17 wherein said second branch of the current mirroring circuit directly feeds the biasing current to said integrated comparator circuit.
 23. The circuit of claim 20 wherein said means to modify the current mirroring ratio is built by an arrangement of FET-Transistors, which operates as a voltage controlled resistive element. 24 The circuit of claim 20, wherein the smaller one of the two said input signals connects to one of the circuits, building a voltage controlled resistive element and in series with one of the current mirror branches, and wherein the larger one of the two said input signals connects to the other of the circuits, building a voltage controlled resistive element and in series with the other branch of the current mirror.
 25. A circuit to supply the biasing current for a comparator with overall reduced power consumption and where the biasing current has its maximum when the voltage difference of the input signal is high and has its minimum when the voltage difference of the input signal is low, comprising: a CMOS comparator to compare the voltage of two input signals and switch the output according to the polarity of said two input signals' difference; a reference current source; a current mirror, typically built by pairs of transistors, to mirror the reference current into a second current branch; a direct connection to feed the current of said second branch of the current mirror into said comparator circuit as the comparator's biasing current; a first voltage controlled resistive element to modify the current mirroring ratio, in series with one of the said current mirror branches and controlled by one of the comparator's inputs and a second voltage controlled resistive element to modify the current mirroring ratio, in series with the other of the said current mirror branches and controlled by the other of the comparator's inputs; a connection of said input signals to the circuit, which modifies the current mirroring ratio; a matrix of switching elements, to alternate the connection between said two input signals and the proper branch of said voltage controlled resistors, which modifies the current mirroring ratio of said current mirror; and a circuit to select the status of said switch matrix, to alternate the connection between said two input signals and the proper branch of said voltage controlled resistors, based on the output status of said comparating means.
 26. A circuit of claim 25, where the voltage controlled resistive elements to modify the current mirroring ratio are formed by an arrangement of FET transistors.
 27. A circuit of claim 25, where the matrix of switching elements is formed by an arrangement of FET transistors.
 28. A method of feeding high biasing current to a CMOS comparator only when the input voltage difference is high, comprising: providing a CMOS voltage comparator, means to mirror the reference current into a second current branch and feeding this said current into said comparator circuit as its biasing current, means to modify the current mirroring ratio depending on said input signals' voltage difference, means to connect said input signals to the circuit, which modifies the current mirroring ratio, means to alternate which of the two input signals is connected to which branch of said circuit, which modifies the current mirroring ratio of said current mirror and means to control the status of said alternating mechanism for said two input signals and the two said branches of the current mirroring circuit; comparing the voltage of two input signals with a CMOS circuit and switching the output according to the polarity of said two input signals difference; mirroring said reference current into a second current branch which has either a one to one relationship to said reference current or which otherwise has a fixed ratio to the reference current; feeding said second current into said comparator circuit as the comparators biasing current; modifying the current mirroring ratio, in order to increase said biasing current when said input signals' voltage difference becomes larger; producing an unmodified mirroring ratio when the voltage difference of both input signals is zero, wherein at this point the current mirror is defined to be in balance and said mirrored current is equal to the desired minimum of said biasing current; connecting said input signals to the means that modify the current mirroring ratio depending on said input signals' voltage difference; alternating the connection of each of the two input signals to the proper branch of the means, that modifies the current mirroring ratio, depending on the polarity of said input signals' voltage difference; and selecting the proper status of said connection between said two input signals and the two said branches of the current mirroring circuit.
 29. The method of claim 28 wherein reducing said current mirroring ratio is achieved by reducing the resistance of said first voltage controlled resistive element, which is in series with one branch of said current mirror and by increasing the resistance of said second voltage controlled resistive element, which is in series with the other branch of said current mirror.
 30. The method of claim 28 wherein increasing said current mirroring ratio is achieved by increasing the resistance of said first voltage controlled resistive element, which is in series with one branch of said current mirror and by reducing the resistance of said second voltage controlled resistive element, which is in series with the other branch of said current mirror. 31 The method of claim 30 wherein both voltage controlled resistive elements, which are in series with said two branches of said current mirror, are in balance at the point in time, where the voltage difference of said comparator reaches zero and where said mirrored current is equal to the desired minimum of said biasing current.
 32. A circuit to supply the biasing current for a comparator with overall reduced power consumption and where the biasing current has its maximum when the voltage difference of the input signal is low, comprising: means to compare the voltage of two input signals, where the output switches according to the polarity of said two input signals' difference; a reference current source; means to mirror the reference current into a second current branch; means to feed the current of said second branch of the current mirror into said comparator circuit as the comparator's biasing current; means, to modify the current mirroring ratio, in series with one branch of said current mirror and controlled by signals from both inputs of said comparator, and means, to modify the current mirroring ratio, in series with the other branch of said current mirror and again controlled by signals from both inputs of said comparator; and means to connect said input signals to the circuit elements, which modify the current mirroring ratio.
 33. The circuit of claim 32 wherein said means to compare the two input signals is an integrated voltage comparator circuit.
 34. The circuit of claim 32 wherein said means to mirror the reference current is built by an arrangement of FET-Transistors.
 35. The circuit of claim 32 wherein said means to modify the current mirroring ratio are built by an arrangement of FET-Transistors, which operate as a set of voltage controlled resistors.
 36. The circuit of claim 32 wherein said second branch of the current mirroring circuit directly feeds the biasing current to said integrated comparator circuit.
 37. A circuit to supply the biasing current for a comparator with overall reduced power consumption and where the biasing current has its maximum when the voltage difference of the input signal is low, comprising: a CMOS comparator to compare the voltage of two input signals and switch the output according to the polarity of said two input signals' difference; a reference current source; a current mirror, typically built by pairs of transistors, to mirror the reference current into a second current branch; a direct connection to feed the current of said second branch of the current mirror into said comparator circuit as the comparator's biasing current; a first resistive element to modify the current mirroring ratio, in series with one branch of the said current mirror and connected with one of the comparator's inputs and a second resistive element, parallel to the first resistive element, to modify the current mirroring ratio, in series with the same branch of said current mirror and connected with the other of the comparator's inputs and a third resistive element to modify the current mirroring ratio, in series with the second branch of said current mirror and connected with one of the comparator's inputs and a forth resistive element, in series with the third resistive element, to modify the current mirroring ratio, in series with the second branch of said current mirror and connected with the other of the comparator inputs; and connections of said input signals to the circuits, which modify the current mirroring ratio.
 38. The circuit of claim 37, where the voltage controlled resistive elements to modify the current mirroring ratio are formed by an arrangement of FET transistors.
 39. A method of feeding high biasing current to a CMOS comparator only when the input voltage difference is low, comprising: providing a CMOS voltage comparator, means to mirror the reference current into a second current branch and feeding this said current into said comparator circuit as its biasing current means to modify the current mirroring ratio depending on said input signals' voltage difference and means to connect said input signals to the circuit which modifies the current mirroring ratio; comparing the voltage of two input signals with a CMOS circuit and switching the output according to the polarity of said two input signals difference; mirroring said reference current into a second current branch which is equal or a predefined multiple of to said reference; feeding said second current into said comparator circuit as the comparators biasing current; modifying the current mirroring ratio by the parallel arrangement of two voltage controled resistive elements in one branch of said current mirror and by the serial arrangement of two voltage controlled resistive elements in the other branch of said current mirror, that are controlled trough said comparator input signals; and connecting said input signals to the means that modify the current mirroring ratio depending on said input signals' voltage difference.
 40. The method of claim 39 wherein reducing said current mirroring ratio is achieved by reducing the resistance of said first voltage controlled resistive element, which is in series with one branch of said current mirror and by increasing the resistance of said second voltage controlled resistive element, which is in series with the other branch of said current mirror.
 41. The method of claim 39 wherein increasing said current mirroring ratio is achieved by increasing the resistance of said first voltage controlled resistive element, which is in series with one branch of said current mirror and by reducing the resistance of said second voltage controlled resistive element, which is in series with the other branch of said current mirror. 42 The method of claim 41 wherein said parallel arrangement of two voltage controlled resistive elements in one branch of said current mirror and said serial arrangement of two voltage controlled resistive elements in the other branch of said current mirror are in balance at the point in time, where the voltage difference of said comparator reaches zero and where said mirrored current is equal to the desired maximum of said biasing current. 